PROJECT SUMMARY/ABSTRACT Cryptococcus-associated immune reconstitution inflammatory syndrome (C-IRIS) is induced in immunocompromised patients who were pre-infected with Cryptococcus before recovering immune function. C-IRIS is known to occur in HIV patients who received antiretroviral therapy, which restores immune components such as CD4+ T cells. C-IRIS is also reported in immunocompromised patients who received a solid-organ transplant, multiple sclerosis patients who discontinue Natalizumab treatment, and postpartum women. This suggests that any clinical condition associated with a rapid change in immune status is conducive to C-IRIS. Patients with C-IRIS typically present with pulmonary dysfunction, brain edema, and brain lesions. C-IRIS can be fatal. Currently, the mechanism by which immune system reconstitution induces detrimental signaling in patients with an underlying Cryptococcus infection is poorly understood. To resolve this significant clinical challenge, we established the first reliable mouse model using the Cryptococcus neoformans (Cn) serotype A strain H99. Cn H99 is the most common Cn serotype to be isolated from the environment and from clinical samples. Our model of immunocompromised mice that are pre-infected with Cn H99 and received CD4+ T cell transfer show phenotypes reminiscent of those in C-IRIS human patients. Our C-IRIS mice show an abundance of Cn H99 and CD4+ T cells in the brain, brain edema, brainstem neuronal damage, and pulmonary dysfunction without lung histological damage. Therefore, we hypothesize that 1) Cn H99 in the brain trigger CD4+ T migration to this organ, 2) brain-infiltrating CD4+ T and innate immune cells damage brainstem neurons, which control respiratory function, directly and/or indirectly through edema, and that 3) brainstem neuronal damage leads pulmonary dysfunction and eventual death. Our objective is to identify the mechanisms underlying these key symptoms to pave the way toward the development of targeted clinical therapeutics for C-IRIS. In Aim 1, we will identify the role of CD4+ T cell subtypes and innate immune cells in the brain for C-IRIS development. In addition, we will identify the potential therapeutic strategies by targeting CD4+ T cell and Cn H99 migration to the brain and upregulated co-stimulatory molecules. In Aim 2, we will determine whether damage of lung-innervating neurons in the brainstem leads to pulmonary dysfunction and death in C-IRIS mice. We will also identify the mechanism of brain neuronal damage in C- IRIS mice by targeting four candidate pathways (brain edema, brain CD4+ T cell neurotoxicity, brain innate immune cell neurotoxicity, and brain Cn H99). Completion of the proposed experiments will unravel how immune system reconstitution induces detrimental signaling in C-IRIS and expand targeted therapeutic avenues for C-IRIS. Furthermore, this study will provide mechanistic insights into how adaptive immune responses modulate brain neuronal function. Findings from this study will also open a new area of research to investigate the potential infective component in autoimmune and neurodegenerative diseases.